The present invention relates to a color cathode ray tube and, more particularly, to a structure for supporting a shadow mask on an inner surface of a panel thereof.
As shown in FIG. 1, a conventional color cathode ray tube has a vacuum envelope comprising rectangular panel 1, funnel 2, and neck 3. Phosphor screen 4 consisting of phosphor stripes for emitting red, green, and blue light rays, upon landing of electron beams thereon, is formed on an inner surface of a faceplate of panel 1. So-called in-line electron gun assembly 6 is aligned along the horizontal axis of panel 1, and is arranged in neck 3 so as to emit three electron beams corresponding to the red, green, and blue phosphor stripes. A peripheral portion of shadow mask 5 is supported by mask frame 17. Shadow mask 5 has a large number of slit apertures aligned in the vertical direction and a large number of vertical arrays aligned in the horizontal direction. Frame 17 is fixed at positions near screen 4 through resilient support member 12.
Three in-line electron beams 14 are deflected by external deflection coil 9 located outside funnel 2 and the shadow mask 5 is scanned with the deflected beams. Electron beams 14 pass through the apertures of mask 5 and land on their corresponding phosphor stripes, thereby reproducing a color image. In order to prevent degradation of color purity in the reproduced image, caused by mislanding of electron beams on the phosphor stripes due to an external magnetic field influence such as geomagnetism, magnetic shielding plate 8 of a ferromagnetic metal is locked inside funnel 2 through frame 7.
In such a color cathode ray tube, a pitch of the slit apertures of mask 5 must be about 1/3 that of the phosphor stripes. For this reason, the number of effective electron beams 14 passing through the slit apertures is normally decreased to 1/3 or less. The remaining electron beams 14 bombard mask 5, often heating it to about 80.degree. C. In particular, in special color cathode ray tubes used for display CRTs in aircraft cockpits, shadow masks are often heated to about 200.degree. C. Mask 5 is normally made of a 0.2-mm thick thin plate which has, as a major constituent, iron with a relatively large thermal expansion coefficient. The peripheral portion of mask 5 is fixed by a 1.6-mm thick rigid mask frame 7. Electron beams 14 bombarding mask 5 heat and expand it, thus changing a gap (to be referred to as a Q value for brevity hereinafter) between screen 4 and mask 5. When a change in Q value exceeds an allowable range, electron beams 14 cannot land accurately on the phosphor stripes, thereby causing mislanding and the subsequent color purity degradation described above. In order to prevent this drawback, in a conventional color cathode ray tube described in Japanese Patent Publication No. 44-3547, mask frame is locked on a panel side wall, i.e., a skirt, through bimetal as a resilient support member. When it is heated, the entire mask is moved by bimetal toward screen 4 so as to substantially maintain the Q value within the allowable range.
However, the structure using the bimetal described above is complicated and requires a large number of components, thus varying the dimensional precision of the tube. In addition, since the principle of operation is based on a heat conduction route of the shadow mask, the mask frame and the bimetal, heat conduction is very slow and cannot provide a sufficient correction effect. As a result, the color impurity varies, and a high-quality color cathode ray tube becomes expensive.
Japanese Patent Publication No. 58-144 (Japanese Patent Disclosure No. 53-144252) describes the color cathode ray tube shown in FIG. 2. In this tube, frame 17 of shadow mask 5 is supported by frame support or hook member 12 on the inner surface of panel 1. Support 12 is elastic and deformable and has a substantially V-shaped section. This prior art also describes that electron beam mislanding caused by dooming can be prevented when an angle .theta. of the V-shaped frame support is half that of the deflection angle of the tube. However, in such a color cathode ray tube, as was prepared by the present inventors according to the above conditions, it was confirmed that the degradation of color purity could not be sufficiently prevented.
As shown in FIG. 3, Japanese Patent Publication No. 46-4104 describes a color cathode ray tube with substantially L-shaped resilient support member 12. A mask frame is not used and member 12 is directly connected to the peripheral edge of shadow mask 5. Support member 12 is inclined by .theta.=45.degree., i.e., half of the deflection angle (90.degree.) with respect to the peripheral edge of mask 5. Unlike the color cathode ray tube described in Japanese Patent Publication No. 58-144, heat can be directly conducted from the shadow mask to the mask support. It may be concluded that, with the above structure, the degradation of color purity caused by dooming can be effectively prevented. However, a test by the present inventors confirmed that such degradation was not prevented, and was actually made worse.
In U.S. Pat. Nos. 3,808,493 and 4,482,426, and West Germany Pat. No. 2,231,101, in order to control mislanding of the electron beams in association with thermal expansion of the shadow mask, color cathode ray tubes with a shadow mask of an 36% Ni-Fe alloy, i.e., an invar steel member, are commercially available. The invar steel member has a thermal expansion coefficient as small as 1/10 of that of the conventional cold-rolled steel plate mainly made of Fe and greatly reduces thermal expansion of the shadow mask. However, when an invar steel member is used in the shadow mask, the mask frame is preferably also constituted by an invar steel member, in order to prevent a conventional drawback (e.g., thermal deformation of the shadow mask) caused by a difference between thermal expansion coefficients of the shadow mask and the mask frame during heat treatment. In practice, since the Ni alloy for invar steel members is expensive, characteristics of the color cathode ray tube are improved by the use of an invar steel member as a shadow mask while a cold-rolled steel plate mainly made of Fe is used as the mask frame, in order to minimize an increase in total cost of the color cathode ray tube. In a color cathode ray tube using such an invar steel member, however, the degradation of color purity cannot be sufficiently prevented (to be described in detail later).